The present invention relates to a high-frequency circuit element, such as a resonator, a filter or the like, used for a high-frequency signal processor in communication systems, etc.
A high-frequency circuit element, such as a resonator, a filter or the like, is an essential component in high-frequency communication systems. The main examples of high-frequency circuit elements such as resonators, filters or the like presently used are those using a dielectric resonator, those using a transmission line structure (a microstrip structure or a strip line structure), and those using a surface acoustic wave element. Among those examples, those using a transmission line structure are small and can be applied to frequencies as high as microwaves or milliwaves. Furthermore, they have a two-dimensional structure formed on a substrate and easily can be combined with other circuits or elements, and therefore they are widely used. Conventionally, a half-wavelength resonator with a transmission line is most widely used as this type of resonator. Also, by coupling a plurality of these half-wavelength resonators, a high-frequency circuit element such as a filter or the like is formed (Minute Explanation Examples/Exercises, Microwaves Circuit, Tokyo Electrical Engineering College Publishing Office).
Other conventional examples of a transmission line structure include those using a planar circuit structure. The representative examples are those constructing various high-frequency circuits by using a disc type resonator (Papers of Institute of Electronics and Communication Engineers of Japan, 72/8 Vol.55-B No.8 xe2x80x9cAnalysis of Microwave Planar Circuitxe2x80x9d Tanroku MIYOSHI, Takaaki OOKOSHI).
However, in a resonator having a transmission line structure such as a half-wavelength resonator or the like, high-frequency current is concentrated in a part of the conductor. Therefore, loss due to conductor resistance is relatively large, resulting in degradation in the Q value in the resonator and also an increase in loss when a filter is formed. Also, when using a half-wavelength resonator having a commonly used microstrip structure, the effect of loss due to radiation from a circuit to space is a problem.
In the case of using a circular resonator or the like as a resonator in a planar circuit structure, it is difficult to obtain a high degree of coupling that satisfies a filter design parameter in a coupling part between an inputoutput line and the resonator. The following prior art technique has been proposed as a method for obtaining a high degree of input-output coupling (FIGS. 12 and 13). That is to say, as shown in FIG. 12, a notch 30a is formed in a part of a resonator 30 and the point of an input-output line 31 is inserted into the notch 30a. This enables the degree of input-output coupling to be increased by increasing coupling capacity (T. Hayashi and others, Electronics Letters, Vol. 30, No. 17 pp. 1424). As shown in FIG. 13, the line width of the point 31a of an input-output line 31 is broadened, and the point 31a having the broadened line width is located facing the peripheral part of the resonator 30. This enables the degree of input-output coupling to be increased by increasing the coupling capacitance.
However, even when using these prior art methods, there is a limit to the increase in the degree of input-output coupling. In the former method (FIG. 12), since the notch 30a is formed in a part of the resonator 30, the current is concentrated at this part, thus causing an increase in loss. On the other hand, in the latter method (FIG. 13), the irregularity in impedance is caused by broadening the line width at the point 31a of the input-output line 31. Conversely, when making the line width of the point 31a too broad, the degree of input-output coupling decreases.
The present invention aims to solve the problems mentioned above in the prior art. The object of the present invention is to provide a high-frequency circuit element that can realize a high degree of input-output coupling without causing an increase in loss and irregularity in impedance.
In order to attain the object mentioned above, an aspect of a high-frequency circuit element according to the present invention comprises at least one resonator having a planar circuit structure and at least one input-output line, and is characterized in that the input-output line has a side edge and a part of the side edge of the input-output line is located along a coupling part on the peripheral part of the resonator and spaced from the resonator by a gap part. According to this aspect of the high-frequency circuit element, distributed coupling can be made by locating a part of the side edge of the input-output line along the coupling part on the peripheral part of the resonator, and spaced therefrom through the gap part. As a result, a high degree of input-output coupling can be realized without changing the peripheral shape of the resonator and the line width of the input-output line at the coupling part as in a conventional high-frequency circuit element, that is, without causing an increase in loss and irregularity in impedance.
In the aspect of the high-frequency circuit element of the present invention mentioned above, it is preferable that the input-output line has a substantially uniform width.
In the aspect of the high-frequency circuit element of the present invention mentioned above, a resonator having any shape, such as a round resonator, an elliptical resonator, a polygonal resonator or the like, can be used as the resonator in a planar circuit structure.
In the aspect of the high-frequency circuit element of the present invention mentioned above, the length of the coupling part defines the angle with respect to the center of the resonator. It is preferable that the angle is set in the range of 5-30xc2x0.
In the aspect of the high-frequency circuit element of the present invention mentioned above, it is preferable that the distance between the coupling part on the periphery of the resonator and the input-output line (the gap part) is set in the range of 10-500 xcexcm.
It is preferable that the high-frequency circuit element of the present invention mentioned above has a microstrip structure or a strip line structure. The microstrip structure is simple in structure and has good coherency with other circuits. The strip line structure enables a high-frequency circuit element having small loss to be realized, since the radiation loss is very small.
In the aspect of the high-frequency circuit element of the present invention mentioned above, it is preferable that an elliptical resonator is used as a resonator in a planar circuit structure and two input-output lines are coupled to the resonator, wherein the coupling parts are in the vicinity of the intersections of the periphery of the resonator with the major axis of the ellipse and the minor axis of the ellipse respectively and are provided at the positions about 90xc2x0 apart from each other with respect to the center of the resonator. This preferable example can be operated as a band pass filter. It is conceivable that it can be operated as a two-stage resonator coupled filter by utilizing the coupling between two resonance modes of the elliptical resonator.
In the aspect of the high-frequency circuit element of the present invention mentioned above, it is preferable that a superconductor is used as a material of the resonator. According to this preferable example, a high-frequency circuit element having small loss and excellent power endurance characteristics can be realized.